1. Field of the Invention
The present invention relates to an optical element and an optical device.
2. Description of the Related Art
A transparent substrate with a grating pattern formed on a surface thereof is known as a wave plate having a minute concave and convex structure of a period (subwavelength period) smaller than a light wavelength to be used. Such a wave plate exhibits optical anisotropy (structural birefringence) based on the grating pattern and is capable of generating phase difference relative to two linearly polarized lights whose planes of polarization are orthogonal to each other.
Important characteristics of the wave plate include transmittance and phase difference. Preferably, the wave plate has high transmittance and in addition to the capability of generating a desired phase difference relative to two linearly polarized lights whose planes of polarization are orthogonal to each other.
The above-mentioned phase difference is expressed by functions such as a refractive index of a transparent substrate material, a groove depth of the grating pattern (minute concave and convex structure) having a subwavelength period, and the like. When the refractive index of a transparent substrate material becomes larger or the depth of the groove of the grating pattern becomes deeper, it is possible to generate a larger phase difference.
However, it is not necessarily easy to manufacture a grating pattern having a subwavelength structure with a large groove depth through molding or the like.
In view of this, Patent Document 1 discloses a phase plate prepared by filling the grating pattern formed on the transparent substrate with a medium having a refractive index sufficiently large in comparison with the refractive index of the transparent substrate and forming a grating pattern with the same period as the grating pattern formed on the transparent substrate so as to generate a larger phase difference. In this phase plate, it is possible to realize such a larger phase difference. However, the medium having the refractive index sufficiently large in comparison with the refractive index of the transparent substrate is disposed on the transparent substrate, so that a quantity of light reflected on a surface of the medium having a large refractive index is increased, so that transmittance is reduced.
In view of such a reduction of transmittance, there is proposed a low refractive index film further formed on the medium having the high refractive index filled into the grating pattern of the transparent substrate, the low refractive index film having a refractive index lower than that of a film material for a high refractive index film (Patent Document 2).
In this film, light is projected onto the low refractive index film from the air side, so that it is possible to reduce reflection on a boundary surface between the air and the low refractive index film and reduce reflection on a boundary surface between the low refractive index film and the high refractive index medium. However, a difference of the refractive indexes between the high refractive index medium and the transparent substrate remains large, so that reflection on a boundary surface between the high refractive index medium and the transparent substrate is not reduced.
Patent Document 1: Japanese Laid-Open Patent Application No. 7-99402
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-099099